A Method of Using Two Equivalent Negative Inductances to Reduce Parasitic Inductances of a Three-Capacitor EMI Filter

Capacitors are commonly used as components of electromagnetic interference (EMI) filters. However, the parasitic inductances of capacitors often limit the high-frequency performance of filters and make it difficult to meet EMI requirements. This letter proposes an approach to reduce the parasitic inductances of a typical three-capacitor EMI filter by introducing two equivalent negative inductances from two terminals of three coupled inductors. Theoretical analysis and formula deduction for the design of the two equivalent negative inductances and the reduction of capacitor parasitic inductances are demonstrated. The proposed configuration can further improve the performance of the filter by increasing its order. Verification by numerical simulations and experimental measurements shows that the proposed method can effectively improve high-frequency filter performance. In the case we studied, the insertion losses of a three-capacitor EMI filter at 50 MHz could be reduced by 16.8 dB for the differential-mode and by 19.2 dB for the common-mode components when two negative inductances were applied.